The emission control of CO2 has recently become strict. Accordingly, it is increasingly desired that fuel efficiency of vehicles be increased by reducing vehicle weight, and reducing the thicknesses of automotive parts by using high strength steel sheets. As the high strength galvanized steel sheet is broadly applied, the requirements of formability and surface quality become strict. Accordingly, a high strength galvanized steel sheet prepared by adding a solute-strengthening element to a so-called “IF” steel in which C and N are precipitated and fixed is often used, in view of the formability and the corrosion resistance (Japanese Unexamined Patent Application Publication No. 2007-169739). The surface quality of the galvanized steel sheet may be degraded due to non-uniformity of coating and a coating defect resulting from Fe—Si oxides or Si oxides such as SiO2, precipitated at the surface of the base iron. Also, scale produced during hot rolling may be partially left after pickling and cold rolling and result in non-uniformity of coating. It is known that such a surface defect produced by scale can degrade surface quality. Also, if non-uniform nitridation occurs during annealing, non-uniform deformation may be caused by press forming. Consequently, linear defects may be produced in the surface of the resulting product.
To solve these problems, a semi-ultra-low carbon steel sheet exhibiting high surface quality and superior press formability and a method for manufacturing the same are disclosed (Japanese Patent No. 4044795). Also, a method for manufacturing a hot rolled steel sheet exhibiting high surface quality is disclosed for descaling in a process of hot rolling (Japanese Unexamined Patent Application Publication No. 6-269840).
Furthermore, a method for preventing nitrogen from permeating the steel sheet during annealing is disclosed for preventing nitridation during annealing (Japanese Unexamined Patent Application Publication No. 48-48318).
The technique disclosed in JP '739 is not effective in enhancing the quality of appearance of coated steel sheets.
In the technique disclosed in JP '795, a relatively large amount of C is used. Accordingly, it is required that a large amount of Nb and Ti, which are elements producing carbonitrides, be added to fix C and N in a form of their alloy precipitate. Consequently, nitridation is likely to occur during annealing and result in linear defects after press forming. JP '795 does not also lead to a new finding about surface defects caused by scale.
JP '840 requires reheating at the inlet side of the finishing mill and, accordingly, energy cost is increased. In addition, if scale is trapped during roughing rolling and, thus, a cause of defects exists, the effect of reheating is limited.
JP '318 is intended to prevent low-carbon steel from being nitrided during batch annealing, and does not lead to a finding about the behavior of nitridation of ultra-low carbon and high strength steel sheets during continuous annealing.
IF steel-based high strength galvanized steel sheets thus cannot completely prevent Si oxide from causing non-uniformity of coating or a coating defect, or scale from causing non-uniformity of coating, or cannot prevent nitridation during annealing to produce linear defects after press forming. Thus, satisfying appearance quality cannot be achieved.
It could therefore be helpful to provide a high strength galvanized steel sheet with excellent appearance and a method for manufacturing the same that does not have non-uniformity of coating or a coating defect caused by Si oxide or non-uniformity of coating caused by scale, and does not allow linear defects to be caused after press forming by nitridation occurring during annealing.